In work apparatus such as chain saws, brushcutters, suction/blower apparatus or the like, which are driven by internal combustion engines, intake systems are provided for preparing combustion air for the engine. In the intake systems, a carburetor having a forwardly connected air filter and a filter case is mounted surrounding the air filter on the outside. The combustion airflow is drawn by the engine through the carburetor and the air filter out of the interior space of the filter case. In known arrangements, a combustion air channel is provided leading to the filter case from a discharge opening in a cooling air fan. The combustion air is branched off by means of the take-up opening from the cooling airflow moved by the cooling air fan and is made available via the combustion air channel in the interior space of the filter case for drawing in by suction through the carburetor.
In arrangements of this kind, it is, on the one hand, provided that the dynamic pressure, which is caused by the speed of the cooling airflow, leads to an increase of the air pressure of the combustion air and therefore to an increase of the engine power. On the other hand, a preseparator can be provided in the combustion air channel or, especially, in the cooling air fan in the region of the take-up opening. With this preseparator, a preseparation of foreign particles such as dust particles or the like can take place in the region of the take-up opening. In the intake system, a pre-cleaned flow of combustion air is thereby made available. The air filter, which is connected ahead of the carburetor, is subjected to a correspondingly reduced amount of dirt so that the intervals for filter exchanges can be extended which contributes to an increase of the productivity of the work apparatus. If needed, a simpler, more cost effective filter or a filter having a reduced throughflow resistance can be used because of the reduced amount of dirt.
Matching such an intake system to the operating range in question has been shown to be difficult. The stroke movement of the piston in the internal combustion engine leads to a pulsed intake airflow in the region of the carburetor while via the continuously rotating air fan, an airflow moves which is dependent upon the rpm but is essentially continuous. The interaction of the mutually opposite effects can lead to an excitation of the vibration of the air column in the combustion air channel. This, in turn, can have considerable disadvantageous effects on the intake performance in the region of the carburetor and therefore have associated negative effects on the desired uniform mixture formation. The matching of the engine and carburetor to each other as well as their matching to the design of the combustion air channel, the filter case volume or the like can be difficult. The arrangement of a compensator is complex and does not always lead to the wanted success. Especially, it can happen that a found matching functions only in a narrow operating parameter range outside of which no effective matching is given.
For avoiding such problems, U.S. Pat. No. 5,485,817 discloses an arrangement wherein a filter case has a ventilation opening which is either connected to the atmosphere or to a low pressure zone of the cooling fan. The ventilation opening is intended to avoid an excitation vibration of the air column in the combustion air channel. Here, the air channel should be so dimensioned that an underpressure or an overpressure arises in the air filter case. In order to substantially eliminate the influence of the air filter contamination on the mixture formation, it is conventional to provide a compensation connection which connects a compensation chamber of the carburetor to the clean space of the air filter. For fluctuating pressures in the air filter chamber, a compensation is, however, not possible so that the air/fuel mixture cannot be optimally adjusted.